ORIGIN OF CHANNELS WITHIN BARRIERS. 93 



inner. The extent of reef-grounds within a barrier, raised by 

 accumulations at the same time with the reefs, is often fifty 

 times greater than the area of the barrier itself. Owing to these 

 causes the rate of growth of the barrier may be at least twice 

 more rapid than that of the inner reefs. If the barrier increases 

 twenty feet in height in a century, the inner reef according to 

 this supposition would increase but ten feet ; and any rate of sub- 

 sidence between the two mentioned, would sink the inner reefs 

 more rapidly than they could grow, and cause them to disap- 

 pear. A wide flat reef, continuous over so extensive reef-grounds, 

 could be formed only with an extremely slow rate of subsi- 

 dence ; and even then they would be liable to be cut up by the 

 production of inner currents, destroying growing corals over the 

 interior parts of the coral reef; so that whatever the rate of sub- 

 sidence, the inner portions would grow less rapidly. There is 

 therefore not only no objection to the theory from the existence 

 of wide channels and open seas; on the contrary, their non-exis- 

 tence is incompatible with the mode of action going on. They 

 afford the strongest support to the theory. 



From these considerations it is evident that a barrier reef 

 indicates very nearly the former limits as to the land enclosed. 

 The Exploring Islets, (Feejee chart,) instead of an area of six 

 square miles, the whole extent of the existing land, once covered 

 three hundred square miles ; and the outline of the former land 

 is indicated by the course of the enclosing reef. A still greater 

 extent may be justly inferred. For the barrier, as subsidence 

 goes on, gradually contracts its area, owing to the fact that the 

 sea bears a great part of the material inward over the reefs: and, 

 consequently, the declivity forming the outer limit of the sub- 

 marine coral formation, has a steep angle of inclination. 



In the same manner it follows that the island Nanuku, instead 

 of one square mile, extended once over two hundred square miles, 

 or had two hundred times the present area of high land. Bacon's 

 Isles once formed a large triangular island of equal extent, though 

 now but two points of rock remain above the water. 



The two large islands in the western part of the group, Vanua 

 Levu and Viti Levu, have distant barriers on the western side. 

 Off the north point of the former, the reef begins to diverge from 

 the coast, and stretches off from the shores till it is twenty and 

 twenty-five miles distant ; then, after a narrow interruption, with- 

 out soundings, the Asaua islands commence in the same line, and 

 sweep around to the reef which unites with the south side of 

 Viti Levu ; and tracing the reef along the south and east shores, 

 we find it at last nearly connecting with a reef extending south- 

 ward from Yanua Levu. Thus these two large islands are nearly 

 encircled in a single belt ; and it would be doing no violence to 

 principles or probabilities, to suppose them once to have formed 



